Semiconductor wafers are generally produced by slicing a cylindrical single-crystal or polycrystalline workpiece of the semiconductor material with the aid of a wire saw, simultaneously into a multiplicity of semiconductor wafers in one working step.
The standard components of these wire saws include a machine frame, a forward feed device, and a sawing tool which consists of a gang of parallel wire sections. The workpiece is fixed on a so-called sawing strip, generally by cementing or adhesive bonding. The sawing strip is in turn fastened on a mounting plate, in order to clamp the workpiece in the wire saw.
The wire gang of the wire saw is generally formed by a multiplicity of parallel wire sections, which are tensioned between at least two wire guide rolls, the wire guide rolls being rotatably mounted and at least one of them being driven. The wire sections generally belong to a single finite wire, which is guided spirally around the roll system and is unwound from a stock roll onto a receiver roll.
During the sawing process, the forward feed device induces a relative movement of the wire sections and the workpiece directed against one another. As a result of this forward feed movement, the wire, on which a sawing suspension is applied, works to form parallel sawing kerfs through the workpiece. The sawing suspension, which is also referred to as a slurry, contains abrasive particles, for example consisting of silicon carbide, which are suspended in a liquid. A sawing wire with firmly bound abrasive particles may also be used. In this case, it is not necessary to apply a sawing suspension. It is merely necessary to supply a liquid cooling lubricant, which protects the wire and the workpiece against overheating and at the same time transports workpiece swarf out from the sawing kerfs.
The production of semiconductor wafers from cylindrical semiconductor material, for example from a single crystal, places stringent requirements on the sawing method. The aim of the sawing method is generally that each sawn semiconductor wafer should have two surfaces which are as flat as possible and are mutually parallel.
Besides the thickness variation, the planarity of the two surfaces of the semiconductor wafer is of great importance. After a semiconductor single crystal, for example a silicon single crystal, has been sliced by means of a wire saw, the wafers thereby produced have a wavy surface. This waviness may be partially or fully removed in the subsequent steps, for example grinding or lapping, depending on the wavelength and amplitude of the waviness as well as on the depth of the material removal. In the least favorable case, residues of this waviness may still be detected even after polishing on the finished semiconductor wafer, where they have a detrimental effect on the local geometry. At different positions on the sawn wafers, these waves occur to different degrees. Particularly critical in this case is the end region of the cut in which particularly pronounced waves or grooves may occur, which are even detectable on the end product depending on the nature of the subsequent steps.
From DE102005007312A1, it is known that the wave in the end region of the cut, which occurs in sawing processes according to the prior art, is particularly pronounced for the wafers which have been sliced at the ends of the cylindrical workpiece. In the middle of the workpiece (in the axial direction), on the other hand, the sliced wafers have virtually no wave in the end region of the cut. Furthermore, the axial dynamic pressure gradient generated by the sawing suspension has been identified as a cause of the wave occurring at the end of the sawing process. According to DE102005007312A1, the amount of sawing suspension which is applied to the wire gang is therefore reduced, and the waviness of the sawn semiconductor wafers is thereby reduced in the end region of the cut. It has, however, been found that this measure is not sufficient to satisfy the increasing requirements on the local geometry. In particular, the formation of sawing grooves in the end region is not reliably prevented.
DE102006032432B3 discloses a method in which a sawing strip having oblique side faces is used, in order to reduce the waviness at the end of the cut when the wire passes through not only the workpiece but also the sawing strip. This modified sawing strip also does not prevent the formation of sawing grooves at the end of the cut. Furthermore—particularly in the case of sawing strips composed of a plurality of different materials—additional processing steps are required during the production of the sawing strip, which increases the auxiliary material costs for the sawing process.
Methods are likewise known in which the plane-parallelism of the sawn wafers is improved by varying the workpiece forward feed rate as a function of time. EP856388A2 discloses inter alia a method in which the workpiece forward feed rate is initially reduced as a function of the cutting depth until a cutting depth of about 70% of the workpiece diameter is reached, subsequently reincreased slightly and reduced again at the end. This method makes it possible to produce wafers having a uniform thickness, although the regions of the wafers which correspond to the first and last ten percent of the cutting depth have a significantly smaller thickness. EP856388A2 does not, however, mention any measures for avoiding sawing grooves which specifically occur within the last ten percent of the cutting depth.